1. Field of the Invention
The present invention relates to a high voltage capacitor and magnetron having a filter constituted of the high voltage capacitor.
2. Discussion of the Background
Well known examples of high voltage capacitors of this type in the prior art include those disclosed in Japanese Unexamined Patent Publication No. 316099/1996 and Japanese Unexamined Utility Model Publication No. 40524/1992. They generally adopt a structure achieved in which two through holes are formed over a distance from each other in a dielectric ceramic material constituting the capacitor. Individual electrodes that are independent of each other and a common electrode shared by the individual electrodes are provided at the two surfaces of the dielectric ceramic material at which the through holes open. The common electrode is secured onto a raised portion of a grounding metal by a means such as soldering. Conductors are inserted in the through holes at the capacitor and holes at the grounding metal. The conductors are soldered to the individual electrodes at the capacitor using electrode connectors or the like. An insulating case is fitted at the external circumference of the raised portion of the grounding metal, so as to enclose the capacitor. An insulating cover is fitted at the other surface of the grounding metal, so as to enclose the conductors. The insulating cover is mounted so that it comes into close contact with the internal circumferential surface of the raised portion of the grounding metal. Then, the insulating case and the spaces inside and outside of the capacitor enclosed by the insulating case are filled with a thermosetting insulating resin such as an epoxy resin to assure moisture resistance and good insulation.
However, the insulating cover is simply fitted so that it comes into close contact with the internal circumferential surface of the raised portion of the grounding metal and the insulating cover and the grounding metal are not bonded. As a result, the stress, which repeatedly occurs while the insulating resin is becoming hardened and contracting and during operation of the capacitor causes a gap to form at the boundary of the insulating cover and the internal circumferential surface of the raised portion of the grounding metal and also induces formation of a gap between the insulating resin and the grounding metal and between the insulating resin and the dielectric ceramic material. Thus, high voltage capacitors of this type in the prior art pose a risk of early defects occurring due to deterioration of characteristics and shorting occurring between electrodes.
Main causes of the gap formed between the insulating cover and the grounding metal include the stress occurring while the thermosetting insulating resin such as an epoxy resin is becoming hardened and contracting and the stress attributable to the electrostrictive phenomenon occurring at the dielectric ceramic material constituting the capacitor.
The main constituent of the dielectric ceramic material constituting the capacitor is barium titanate. Such a dielectric ceramic material is a ferroelectric material that belongs to the piezoelectric crystal class. A ferroelectric material belonging to the piezoelectric crystal class imparts a reverse piezoelectric effect. As a result, when a high AC voltage is applied, mechanical energy is generated inside the dielectric ceramic material constituting the capacitor. For instance, if this type of high voltage capacitor is employed in a filter of a magnetron in a microwave oven, a high AC voltage for oscillating the magnetron is applied to the capacitor. When such a high AC voltage is applied to the dielectric ceramic material the reverse piezoelectric effect mentioned earlier converts the electrical energy to mechanical energy. This results in the dielectric ceramic material expanding while the voltage is being applied and contracting to regain its original state when the voltage is not applied. A voltage of approximately 4 kV.sub.O-P having a commercial frequency or a frequency in the range of 20 kHz to 40 kHz is applied to oscillate the magnetron in a microwave oven. In addition, a transient voltage of 0 to 40 kV.sub.P-P is applied immediately before the magnetron starts to oscillate. In response to these AC voltages, the dielectric ceramic material constituting the capacitor repeats a process of expanding and contracting. This is referred to as an electrostrictive phenomenon of a dielectric ceramic material.
However, since the insulating cover is merely fitted in close contact with the internal circumferential surface of the raised portion of the grounding metal, the insulating cover and the internal circumferential surface are not bonded. This causes a gap to form at the boundary of the insulating cover and the internal circumferential surface of the raised portion of the grounding metal while the dielectric ceramic material constituting the capacitor repeats expanding and contracting in response to the AC voltage and, furthermore, induces separation at the interface of the insulating resin and the grounding metal.